A requirement which is made of internal combustion engines and, in particular, of diesel engines is the adherence to limit values for nitrogen oxide emissions, decreasing legal limit values requiring a reduction in said nitrogen oxide emissions. One possibility for reducing nitrogen oxide emissions is feeding parts of the exhaust gas into the combustion chamber, as a result of which an oxygen concentration in the combustion chamber of the cylinder can be set and the peak combustion temperature, in particular in the case of cooled exhaust gas recirculation, can be lowered. Said so-called exhaust gas recirculation (EGR for short) is known and represents one possibility to reduce the oxygen concentration in the combustion chamber of the cylinder. Here, the precise setting of the oxygen concentration in the cylinder during transient operation and also during steady-state operation is of central importance. If the steady-state operation of an internal combustion engine does not make any great requirements of a control system, exhaust gas recirculation has to be determined as precisely as possible, in particular, in transient operation, in order for it to be possible to produce a satisfactory correlation of the nitrogen oxide emissions.
In the case of exhaust gas recirculation, a distinction is made between internal and external exhaust gas recirculation. The return flow of exhaust gas out of the outlet duct into the combustion chamber during a valve overlap phase is called internal exhaust gas recirculation. Here, the level of the internal exhaust gas recirculation depends on a pressure difference between the fresh air feed and the exhaust gas system, on the duration of a valve overlap, and on the opening cross sections which are released by the valves. A further known exhaust gas recirculation is the external exhaust gas recirculation. A distinction is made here between a high-pressure exhaust gas recirculation which makes direct recirculation of the exhaust gases in the region of the exhaust gas manifold into the fresh air feed of the inlet duct possible, and a low-pressure exhaust gas recirculation, in which the exhaust gas is branched off downstream of a turbine (integrated into the exhaust gas system) of a turbocharger and is fed into the fresh air feed upstream of the compressor of the turbocharger.
Exhaust gas recirculation systems of the type mentioned at the outset, with low-pressure exhaust gas recirculation, are disclosed by DE 10 2010 025 699 A1, DE 10 2010 027 646 A1 and WO 2010/072227 A1.
DE 10 2010 025 699 A1 discloses a diesel engine having means for exhaust gas aftertreatment and means for exhaust gas recirculation. An air intake section and an exhaust gas section are provided, at least one soot particulate filter being arranged in the exhaust gas section and an NOx storage catalytic converter being arranged downstream of the latter. An exhaust gas recirculation section branches off from the exhaust gas section and opens into the air intake section. A branch for separating off an exhaust gas part flow is arranged for the exhaust gas recirculation in a housing of the NOx storage catalytic converter. As a result, the NOx storage catalytic converter is not loaded with an excessively great exhaust gas stream, by the exhaust gas part stream for exhaust gas recirculation being precluded to this extent from the exhaust gas aftertreatment. As a result of a thermal transfer between the branch for the exhaust gas recirculation and the housing parts of the NOx storage catalytic converter, furthermore, more rapid heating of the NOx storage catalytic converter advantageously occurs during operation, with the result that the pollutant emissions can be reduced, in particular, in the starting phase of the internal combustion engine.
DE 10 2010 027 646 A1 discloses an exhaust gas system of an internal combustion engine, having an exhaust gas section and exhaust gas recirculation, the exhaust gas recirculation branching off from the exhaust gas section, and the exhaust gas section having a pipe and exhaust gas treatment means. The pipe and/or the exhaust gas treatment means are/is configured at least in sections in such a way that a plurality of lines for exhaust gas are formed, a first line conducting a main exhaust gas stream and a second line conducting an exhaust gas part stream which is branched off for exhaust gas recirculation.
WO 2010/072227 A1 discloses an exhaust gas recirculation system for an internal combustion engine having an exhaust gas turbocharger, an exhaust gas recirculation line being connected firstly for the removal of exhaust gas to an exhaust gas section of the internal combustion engine and being connected secondly to a fresh air feed of the internal combustion engine. The exhaust gas recirculation line is routed via an exhaust gas compressor in order to increase the pressure of the recirculated exhaust gas. As a result of the increased pressure of the recirculated exhaust gas stream, the latter can advantageously be controlled in an improved manner. To this end, a moderate pressure increase can already be sufficient. The pressure level, to which the recirculated exhaust gas is compressed, depends substantially on how and where it is to be fed to the fresh air feed.
In the case of the known low-pressure exhaust gas recirculation systems, in particular in the case of exhaust gas turbocharged internal combustion engines, in particular gasoline engines, that is to say in the case of an introduction of the exhaust gas recirculation upstream of the compressor of the turbocharger, the result is a high dead volume between the EGR feed-in point and the throttle valve, which dead volume is filled with recirculated exhaust gas. In the case of a rapid load reduction, for example as a result of spontaneous decoupling, problems with the combustion stability can occur, since the EGR rate cannot be reduced rapidly enough as a result of the high dead volume and cannot be adapted rapidly enough to the new engine load point.
It is an object of the present invention for it to be possible to adapt the EGR rate more rapidly to changed engine load points.